This application relates to the forming of batteries including tetrabasic lead sulfate plates. In particular, the application relates to the addition of a lead peroxide such as lead dioxide to the electrode paste to allow forming of battery plates in the battery container itself. The use of tetrabasic lead sulfate, chemical formula 4PbO.PbSO.sub.4, has been determined to be a useful starting material for the preparation of electrodes in use in lead-acid batteries. As disclosed in U.S. Pat. No. 3,765,943, issued to Biagetti on Dec. 9, 1970, entitled "FABRICATION OF LEAD-ACID BATTERIES," and in U.S. Pat. No. 3,899,349, issued to Yarnell on Aug. 12, 1975, entitled "CARBON DIOXIDE CURING OF LEAD-ACID BATTERIES," hereby incorporated by reference, battery plates or electrodes fabricated with tetrabasic lead sulfate produce an increased product yield, greater life, greater reproducibility in the chemical and physical properties of the product, and crystal morphology more suitable to battery operation. Such characteristics are most desirable in batteries intended for float or standby service, where extremely long life, in excess of thirty years, is desired.
It should be noted that both positive and negative electrodes may be formed with tetrabasic lead sulfate, although it is preferred that tetrabasic lead sulfate be used only for positive electrodes, because of the greater energy required to form negative electrodes with tetrabasic lead sulfate as a starting material, and because conventional negative electrodes have generally satisfactory characteristics, in terms of growth and shedding of active material, etc.
In forming or charging a battery, the desired result is a positive plate or electrode having lead dioxide as the active material, and a negative plate or electrode having metallic sponge lead as the active material. For negative plates or electrodes, it has been found more practical to start with a mixture of lead and lead oxides, known as "leady oxides" containing approximately 30 percent metallic lead particles.
The preparation of active electrodes or plates for lead acid batteries involves three general steps. The first, a paste is applied to a supporting grid, the pasted grid is cured and dried, and the cured grid is formed to produce the active material used in the charged lead-acid battery. Formation is usually performed by electrolytic formation, by passing current through the electrode, to cause the material of the electrode to react with the electrolyte to form the desired materials in the electrodes. This process is skin to charging a battery in use, except that the initial formation of the desired active materials requires much greater care to produce a desirable structure for the battery plates. The desired end result is a negative plate or electrode having pellets of sponge lead, and a positive plate or electrode having pellets of lead oxide.
It has been stated that the only parameters relevant to mixing of paste for tetrabasic lead sulfate plates or electrodes are the amounts of tetrabasic lead sulfate and water, and the length of time mixed, by C. F. Yarnell, Abstract No. 32, issued October, 1974, New York, Electrochemical Society Meeting.
When the water is first added, the mix appears very dry. After continued mixing, the mixture starts to form into small balls, the small balls coalescing into larger balls until the mixture becomes a single mass. As the paste is mixed beyond this point, it becomes more fluid, eventually becoming too fluid to use, regardless of the amount of water used to make the paste, as long as a minimum amount is used.
The paste is then applied to an electrode, typically in the form of a grid work formed of metallic lead or a lead alloy. The pasted plate or electrode is then dried and cured.
Curing may be performed in the manner shown in U.S. Pat. No. 3,899,349 above, or in accordance with U.S. Pat. Nos. 4,331,516 and 4,338,163, in that powdered ammonium carbonate would then be added to the paste constituents.
Then, conventionally, the plates or electrodes are formed to convert the tetrabasic lead sulfate to lead dioxide, for a positive plate, prior to assembly in forming tanks. A fully formed battery may have any desired electrolyte specific gravity. However, it was believed that tetrabasic lead sulfate plates could not be formed practically in the presence of an electrolyte having a high specific gravity.
Positive plates made from tetrabasic lead sulfate paste are conventionally formed, prior to assembly, in a bath of electrolyte having a specific gravity ranging from 1.005 to 1.015, a process requiring between sixty and seventy-two hours at a conventional current of 9 amperes per plate. The time and current figures are typical for plates for a commercially-available cylindrical cell. If the electrolyte specific gravity is increased beyond 1.015, the forming rate diminishes rapidly, such that using an electrolyte specific gravity of 1.050 requires a forming time of two hundred and eighty to three hundred and forty hours. Since a much longer time is required at the same current, it will be apparent that much energy is wasted, a high specific gravity process consuming nearly five times the energy of a low specific gravity process. At the same time, a heavy coating of lead sulfate is formed on the plate surface. This lead sulfate coating falls from the plate surface, and deposits a layer of sludge in the formation vessel.
As is known, there is some sulfuric acid effectively present in all such unformed plates. In conventional plates, sulfuric acid is added when mixing the plates, making them self-curing. In tetrabasic lead sulfate plates, the sulfuric acid exists in the form of sulfate (SO.sub.4) loosely bound to the tetrabasic lead. When placed in a small volume of electrolyte, such as in the final container or jar, this fixed amount of acid also results in a high initial specific gravity.
These problems of sludge formation and energy inefficiency thus effectively preclude formation of the battery plates in their final container, since the excessive sludge would virtually assure premature failure due internal shorting, while the wasted energy, in the form of heat, would readily generate temperatures in the confined area of the final container, commonly referred to as a jar, capable of damaging the cell.
Therefore, tetrabasic lead sulfate battery electrodes are conventionally formed in forming tanks of large volume, so that inherent sulfuric acid in the plates cannot cause a high electrolyte specific gravity, with larger heat dissipation area, and where the lead sulfate sludge can be removed. After forming, the electrodes are removed and stored pending assembly into finished batteries. Inherent in the process is exposure to lead dust, sulfuric acid and acid mist to those working around the forming tanks and inserting and removing the electrodes, and also in disposing of waste acid and cleaning the sludge from the formation tanks. The forming electrolyte is normally discarded, due to presence of suspended lead sulfate. This combination of caustic acid and toxic metal presents a costly problem of safe disposal. This process also creates other problems, in that reliable connections between electrodes in the forming tank are difficult to insure, and that the formed electrodes must be kept from drying unevenly before assembly into a battery and final charging. The process of assembling the battery adds yet another opportunity for worker exposure to acid and lead dust.